In some speed sensing applications, it is important to carefully control the position of the wheelshaft regardless of the rotation speed of the wheelshaft. Some conventional approaches use magnetic sensors based on the Hall effect to detect the magnetic field generated by a magnetic encoder wheel mounted on the shaft to be controlled. Others use sensors based on magnetoresistive effects, including the giant magnetoresistive effect (GMR), anisotropic magnetoresistive effect (AMR) or the tunneling magnetoresistive effect (TMR).
In magnetoresistive systems, a differential Wheatstone bridge-like approach typically is used to avoid issues related to thermal drift. In AMR systems, this can be done by tilting the current flow +/−45 degrees with respect to the encoder field axis. In typical spinvalve-like GMR or TMR systems, this can be achieved either by locally different magnetization, which is quite difficult to be realized and can require compromises in signal amplitude, or by spatial separation of the resistive elements. The latter works well if the spacing between the two halves of the bridge corresponds to half of the polewheel pitch. Any deviation from this pitch match will lead to a degradation of the differential signal amplitude and an increase in signal jitter, among other potential negative effects. Another general weakness of conventional monocell approaches is a lack of information regarding the direction of rotation of the encoder wheel.
Therefore, there is a need for improved xMR sensors.